1. Field of the Invention
The present invention relates to a semiconductor laser structure which ensures stable operation of a semiconductor laser element.
2. Description of the Prior Art
A semiconductor laser having an index guide structure has widely been used as a light source in an optical information processing system such as an information recording/drawing disc system. In such a system, if a return beam reflected from the disc surface is again introduced into the semiconductor laser, the return beam creates an interference effect, thereby increasing noise components as shown in FIG. 1. Further, if the semiconductor laser of the above-mentioned type is disposed in a high temperature ambience while maintaining the laser output intensity constant, noise components extremely increase above a certain temperature as shown in FIG. 2.
In order to resolve these problems, several improvements have been proposed. In accordance with one improvement, a current injection width, namely a stripe width, is narrowed to about the length of the carrier diffusion in an active layer, that is, about 2 to 4 .mu.m so as to minimize the return beam noise and the mode competition noise. In such a semiconductor laser, the laser beam distribution is determined by the gain distribution. Since the cavity volume is small, the spontaneous emission beam greatly influences on the laser mode. Further, the gain spectrum width is widened due to the high density current injection, and the laser emission is performed in the multi longitudinal mode, whereby the influence caused by the return beam and the mode competition effect caused by the temperature rise are minimized. However, in the semiconductor laser having the gain guide structure, the near-field pattern changes due to the variation of the injection current, secular change, or due to the return beam. Therefore, a stable coupling with an optical system is not ensured. Further, the astigmatism is considerably large and, therefore, a good coupling is not ensured with an optical system, especially with a lens system.